This invention relates to a process for the recovery of organic dibasic acids from an aqueous acid solution containing nitric acid. More particularly, the present invention relates to a process for the recovery of useful chemicals from an adipic acid bleed stream containing nitric acid, adipic acid, glutaric acid, succinic acid, and an oxidation catalyst, for example copper and vanadium salts.
Adipic acid is a valuable and widely used chemical. A well-known commercial method of producing adipic acid involves oxidizing mixtures of cyclohexanol and cyclohexanone with nitric acid in the presence of a catalyst, for example, a mixture of copper and vanadium salts. As a by-product of the process there is obtained an aqueous mixture of dicarboxylic acids containing mainly adipic acid, glutaric acid, succinic acid, nitric acid and catalyst. This by-product mixture is taken as a bleed stream on the adipic acid process primarily for the removal of organic acids having less than six carbon atoms. Difficulties have hitherto been encountered in purifying this adipic acid bleed stream and it is usually wasted, causing a formidable environmental problem.
U.S. Pat. No. 3,359,283 issued Dec. 19, 1967, to Campbell et al., is directed to a process of treating a mixture of organic dibasic acids including succinic acid, glutaric acid, and adipic acid, which comprises the steps of heating said mixture at a temperature of at least 190.degree. C. under a reduced pressure for a time sufficient to dehydrate the succinic acid to succinic anhydride, distilling the so-produced succinic anhydride together with the water of dehydration, and recovering the succinic anhydride from the make of the distillation. The patent further discloses that nitric acid present in the crude mixture of organic dibasic acids can be distilled by passing the mixture through a steam still at a temperature between 90.degree. and 150.degree. C. and at a pressure of 10 to 400 mm. of Hg, wherein much of the nitric acid and water is passed overhead. The removal of nitric acid is continued without complete evaporation to dryness taking place until a pH after dilution of at least 1.2 but preferably not greater than 2.2 is obtained.
Although the process of U.S. Pat. No. 3,359,283 is of major interest in this art, I have found that low pressure distillation of nitric acid from the organic dibasic acids is relatively expensive and not very efficient in terms of nitric acid distillation. Moreover, runaway reaction of the nitric acid with the organic dibasic acids may occur if the nitric acid is not efficiently and completely removed from the concentrated reaction mixture. Accordingly, I have carried out considerable research to discover a process whereby nitric acid may be safely and efficiently distilled from organic dibasic acids.